That smoking has a negative impact on health is no longer queried. In addition to the well known risks of lung cancer, smoking is also an independent predictor of cardiovascular morbidity, mortality and development of myocardial infarction [1
]. Smoking may also accelerate cerebral atrophy, perfusional decline and white matter lesions [2
]. In contrast, nicotine has plausible mechanisms for aiding cognitive function. Throughout the cholinergic system there are nicotinic acetylcholine receptors which can bind to nicotine. Use of nicotine as an agonist is said to up regulate these receptors in a dose dependent fashion, possibly by several hundred percent, depending on brain region [3
]. The use of nicotine has also been found to aid attention, reaction time and some learning and memory [4
]. Although this effect may be short lived if exposure is discontinued, potential benefits may be conferred as such receptors are thought to decline with ageing, with Alzheimer's disease and Lewy Body dementia [4
]. A histopathological study in this area found mixed results with a possible protective effect of smoking against senile plaque formation in twenty eight matched pairs, and a positive correlation between the amount of smoking and neurofibrilliary change, but in smokers only [5
A Cochrane review attempted to investigate the use of nicotine in Alzheimer's disease, but found no suitable data [6
]. Other authors have compared smokers and non smokers in case control studies of Alzheimer's disease or cross sectional population studies and found mixed results. For example, studies have found no difference between smokers and non smokers in white or African American populations [7
]; significantly increased risk of current smoking (OR2.33) in a Chinese population [8
] and a reduced risk in male smokers only, (but in an unmatched case control study) [9
]. Still further studies found male and female differences [10
], an association between current smoking and reduced psychomotor speed, cognitive flexibility [11
] and no difference between smokers and non smokers aged over fifty years for memory, reasoning and simple choice reaction time [5
]. The smokers however, did die earlier.
One of the difficulties with research in this area is the potential overlapping of risk and protective factors as well as the undoubtedly negative constituents in cigarette smoke, despite smoking remaining the primary means by which people gain nicotine. Other issues include the clustering of risk or protective factors, the possible cognitive protection from a healthy diet [13
], the influence of drinking alcohol and obesity. One study found that smoking was associated with reduced risk of Alzheimer's disease in drinkers only [14
], and in seventy year olds both smoking and obesity were associated with poorer cognition on a neuropsychological battery [16
]. Smoking behaviour may also change over time and by sector or cohort within the population; for example, smoking rates may be markedly lower with increasing age, lower in women [17
] and may be impacted upon by ill health or change in circumstances. Studies may also vary in quality with both matched and unmatched case control studies and much cross sectional data in the literature. In addition to this subjects, either non smokers or smokers, may self select for studies [18
] and survival biases may be present with smokers dying younger [19
As smoking may have a negative effect on the cardiovascular system, it may plausibly impact on the development of dementias, both Alzheimer's Disease and vascular dementia. Several issues combine to make the study of different dementias and smoking important, the impact of these dementias in society, the difficulty in identifying pure forms of either, the increased risk of dementia and cognitive decline in the elderly and the ageing population. All of this combines to focus on the need for an increased understanding of risk and protective factors, particularly if these can be impacted upon via public health messages.
The most recent systematic review is wide ranging and was published by an Australian group in June 2007. The authors examined data from the start of the literature databases 'Pubmed', 'Psychinfo' and 'Cochrane CENTRAL' to June 2005 [20
]. It focused on those over the age of 65 which is pertinent since the risk of dementia increases strikingly with increasing age. This review included only longitudinal studies and found that current smokers relative to never smokers were at increased risk of Alzheimer's disease, Vascular dementia, any dementia and cognitive decline. Former smokers had lower risks than current smokers at least for Alzheimer's disease and cognitive decline [20
]. In parallel to their observations we carried out a similar systematic review using narrower search terms, wider inclusion criteria and more modern literature, including longitudinal studies published between 1995 and 2007 and identified using the literature databases Embase, Psychinfo and Medline. Our review was aimed to examine the relationship between smoking, dementia and cognitive decline in an elderly population.